In recent years, telephone communication systems have expanded from traditional plain old telephone system (POTS) communications to include high-speed data communications as well. As is known, POTS communications include the transmission of voice information, control signals, public switched telephone network (PSTN) information, as well as, information from ancillary equipment in analog form (i.e., computer modems and facsimile machines) that is transmitted in the POTS bandwidth.
Prompted largely by the desire of large businesses to reliably transfer information over a broadband network, telecommunications service providers have implemented digital subscriber line (DSL) to provide a plethora of interactive multi-media digital signals over the same existing POTS twisted-pair lines. Since the introduction of DSL, several major types of DSL service have been developed and deployed. These major types include ISDN DSL (IDSL), Symmetric DSL (SDSL), Asymmetric DSL (ADSL), and High bit rate DSL (HDSL). With the advent of these major types, represented by the aforementioned acronyms, DSL is also referred to as xDSL.
In order to maintain the reliable operation of DSL communications service, the capability to test and evaluate the DSL line, i.e. the twisted-pair lines (which are typically composed of copper), is desired. In some xDSL deployments, a number of Incumbent Local Exchange Carriers (ILEC's) and Competitive Local Exchange Carriers (CLEC's) have been installing additional external devices known as metallic (e.g., copper) “cross-connects” in conjunction with other additional devices known as DSL Access Multiplexers (DSLAM's) to provide metallic access to the DSL line for testing purposes. Testing of the DSL lines for fault detection or evaluation of the bit-rate capacity of a particular loop can be accomplished using cross-connects and DSLAM's to by-pass the DSL line to an integrated test head. Also, functions for trouble-shooting and installation activities on a DSL line are obtained using cross-connects and DSLAM's. But, metallic cross-connects are external devices that are installed in addition to the required devices for normal operation of a communications system. DSLAM's are also additional devices that are typically integrated with the normal system devices, but may also be installed externally. Because of the additional devices and installation requirements, the use of cross-connects and DSLAM's for testing purposes is an undesirably expensive practice.
HDSL/T1 based communications systems are one popular example of the application of xDSL deployments. In HDSL deployments, such as HDSL/T1 based communications systems, current test systems only offer the capability for in-band (i.e. within the system unit) testing. HDSL/T1 based communications systems have evolved in popularity as a result of the development of the HDSL market as a replacement for conventional T1 systems, which consist of dedicated high-speed digital communications circuits. Specifically, HDSL plugs (where a plug contains some number of connection ports) are being integrated into existing T1 systems as an alternative to traditional T1 plugs. Advantages of this practice include the reduction of overhead equipment, such as repeaters (which amplify or regenerate signals to extend transmission distances), improved performance with respect to crosstalk (i.e. interference from adjacent lines), and higher quality bit-error performance. But, since current testing systems for HDSL/T1 based systems only offer in-band testing capability, the capability to test the physical DSL line using such test systems is lacking. Furthermore, this lack of capability to test the DSL line is a deficiency found in current test systems for other types of xDSL communications systems deployments as well, and costly work-arounds have been currently employed, as discussed above.
Expanding on HDSL/T1 based communications systems as an example of current testing practices in xDSL deployments, FIG. 1 shows a simplified block diagram of a typical HDSL/T1 based communications system 100 and related typical testing components 106, 112, as is known in the prior art. In this regard, the communications system includes a central office (CO) line unit 102 and a remote unit 104. The CO unit 102 and the remote unit 104 are networked to each other by one or more DSL lines 110 and to other communications systems (not shown) by T1 circuits 116. The CO unit 102 includes, in addition to the testing components 106, 112, HDSL/T1 interface circuitry 114 and a T1 line interface unit (LIU) 118. Although not shown, the remote unit 104 includes similar components to the CO unit 102, such as interface circuitry 114 and T1 LIU 118.
The testing components 106, 112, only offer the capability for in-band testing of the communications system 100. Essentially, various loop-backs 106 (where a loop-back is a device that redirects a transmitted signal back to the transmitter for testing purposes), are employed within the communications system 100 for testing purposes, as shown in FIG. 1. Testing is accomplished by detection of loop-back control signals transmitted in-band by a loop-back detector, such as the loop-back detector 112. The loop-backs 106 and the loop-back detector 112 enable the locating of a problem in the system 100 at either the CO unit 102 or the remote unit 104, but problems at the remote unit 104 can only be detected when the interfacing DSL line 110 is functioning properly (i.e., acceptable bit-rate capacity, no faults, etc.). Furthermore, the typical testing components 106, 112 do not offer the capability to test the DSL line 110 for faults, proper performance, or other testing criteria.
Therefore, there is a need for a testing system and method capable of testing a DSL line in an xDSL communications system deployment. Furthermore, there is a need for a system and method capable of testing a DSL line in an xDSL deployment that does not require additional, external test-support devices and that is, therefore, cost-effective over the prior art.